1. Field of the Invention
The present invention relates to a focus detecting device for a camera and more particularly relates to a focus detecting device of the type which detects the focusing condition of a camera objective lens on a target object to be photographed, by measuring the light coming from the target object and traversing the objective lens.
2. Description of the Prior Art
A proposed camera focus detecting device of the abovementioned type includes a pair of photocell arrays disposed at positions before and behind a plane that is optically equivalent to a predetermined focal plane e.g. the film plane of the camera, to measure contrasts of the images at the positions.
(1) As the light can not transmit the photocell array, the rear photocell array positioned behind the plane is shadowed by the front photocell array positioned before the plane. As a result, the rear photocell array cannot receive the light from a target object and cannot detect the contrast of the image formed thereon. To avoid this, the conventional focus detecting device had to employ a half-mirror to split the light beam and form two optical axes that are optically equivalent to each other and on which the photocell arrays are disposed respectively. However, since each photocell array is disposed on and perpendicularly with each optical axis, the optical system of the device required much space in the direction perpendicular to each optical axis. PA1 (2) Since the proposed device can detect only whether the objective lens is at an in-focus position and on which side of the in-focus position the same is when it is at an out-of-focus position, the device cannot provide any quantitative data such as how much distance the objective lens should be shifted to reach an infocus-position. Accordingly, in the case when the focusing of the objective lens is automatically adjusted in accordance with the output of the known focus detecting device, the focusing condition must be detected with the objective lens being moved until the in-focus condition is detected whereupon the objective lens is stopped. However, in the case where the objective lens is moved at a high speed for quick response to the operation, the objective lens has moved beyond its in-focus position when the in-focus condition is detected. Thus it is difficult to attain high speed automatic focusing with such a known device.